Wet back boiler



Oct. 12, 1965 F. A. LOEBEL WET BACK BOILER Filed Sept. 20, 1963 INVENTOR. FREDERICK A. LOEBEL BYMW, n

United States Patent 3,211,134 WET BACK BUTLER Frederick A. Loebel, Milwaukee, Wis., assignor to Cleaver-Brooks Company, a corporation of Wisconsin Filed Sept. 20, 1963, Ser. No. 310,437 4 Claims. (Cl. 122-149) This invention relates to boilers and especially boilers of the wet back type. More particularly, this invention relates to a new and improved boiler structure providing a novel flow path arrangement.

Wet back boilers are known in the art. In one form, the wet back boiler includes a fire tube which passes through the chamber containing the liquid to be heated, e.g. water, and the fire tube terminates short of the chamber end but is braced at its terminating end against the chamber end. A flow channel is defined between the end of the furnace tube and the water chamber end, usually between support members extending from the fire tube to the chamber end, so that water is free to circulate over the end or back of the fire tube. The chamber at the back of the fire tube is the wet back chamber and the water or other liquid therein, in addition to absorbing a greater amount of heat in heat exchange with the fire tube, also serves to, in part, insulate the chamber walls from the heat of the fire tube. One of the major advantages of the wet back boiler over the dry back boiler is that it is not necessary to include refractory material in the fire tube end.

However, certain disadvantages exist in the wet back type of boiler. For example, during operation the fire tube is normally at a much higher temperature than the water chamber shell and expands a greater amount. Attachment of the furnace end to the water chamber end adjacent the wet back, although it stabilizes the fire tube,

permits transfer of force of expansion from the fire tube directly to the chamber end. Often, the chamber end is either a tube sheet or an end cap.

The internal pressure of the boiler requires either the use of a multitude of stays where the chamber end is a generally flat plate or else requires the use of a dished plate fore and aft of the boiler chamber for resisting such internal pressure.

It is an object of the present invention to provide a new and useful boiler structure of the wet back type.

A further object is to provide such a boiler structure utilizing a floating end heating element which is constructed and mounted so as not to transmit forces of expansion to the chamber end, e.g. to a tube sheet or door assembly at the end of the boiler chamber.

Still another object is to provide flexibility between the furnace tube or fire tube and the boiler shell so that the fire tube may elongate independently of the shell.

Still another object of the present invention is to provide for the use of a portion of the flow path structure in such a boiler for supporting the tube sheets against pressure within the boiler; it is also the object of the invention to provide a new and useful boiler structure which permits use of flat door assemblies and tube sheets without undue bracing in a wet back design. Other objects and advantages of this invention will be apparent from the following description and from the drawings in which:

FIGURE 1 is a cut-away perspective view of a boiler exploded lengthwise to more clearly show the arrangement of parts;

FIGURE 2 is a view from the end of a boiler similar to that shown in FIGURE 1 with the hinged front door assembly open to show internal structure; and

FIGURE 3 is a cross-section through a modified form of return conduits for use in the boiler illustrated in FIG- URE 1.

While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail an embodiment of the invention together with a modification thereof, with the understanding that the present disclosure is to be considered as an exemplification of the principles of the in vention and is not intended to limit the invention to the embodiment or modification illustrated.

Turning first to FIGURES 1 and 2, there is illustrated a form of boiler provided by the present invention. Accordingly, a base in the form of cross-braced skids 11 is provided for supporting the boiler from a floor or other support structure. A pair of cradle brackets 12 are secured to and project upwardly from base 11. Cradle brackets 12 support the boiler on base 11 with the boiler being slidable for expansion and contraction of the boiler lengthwise.

Shell 14 has a water inlet (not shown) for charging water thereto and is also provided with a steam or hot water outlet 15 for withdrawal of heated fluid. A combustion gas exhaust 16 communicates with the combustion gas flow path within shell 14 for discharge of combustion gases at the end of their flow path. Suitable fittings may be attached at ports 17 as desired, e.g. in the form of pressure gauges, plugs or the like.

The outer casing of the boiler is constructed of shell 14, front door assembly 20 and a rear door assembly 22. Each of door assemblies 20 and 22 is hinged, as indicated at 21 for front door assembly 21), and may be opened and closed in the normal manner for hinged boiler door assemblies. For this purpose, each end of shell 14 is provided with an out-turned circumferential flange 23. Door assembly 20, being in two pieces as will be apparent hereinbelow, is provided with two flanges 23 Bolt holes 25 are provided in each of flanges 23 and 24 for bolting flanges 23 to flanges 24 with door assemblies 20 and 22 in closed position. A mass of shallow insulation 26 is provided on the interior of the rear door assembly 22.

Secured by peripheral flanges within shell 14 are the front tube sheet 27 and rear tube sheet 28, which span the interior of the shell. The boiler chamber 29 is defined within shell 14 between the front and rear tube sheets. The water inlet communicates with chamber 29, as does outlet 15.

Extending through and between the opposing tube sheets 27 and 28 are a plurality of boiler tubes 31. Boiler tubes 31 constitute the major structural members for securing tube sheets 27 and 28 against pressure within chamber 29. A fire tube 32 is secured through tube sheet 27 and a burner 33 is provided for supplying combustion gases to fire tube 32.

The end of the boiler illustrated in FIGURE 2 includes a pair of doors which together form an air chamber for conducting air at superatmospheric pressure to burner 33. The inner door 34 closes directly upon the flange 24 of shell 14 and includes an opening 35 through which burner 33 extends when the doors are closed upon the shell. The outer door 36 is cup-shaped so that when it bracket 38 directly connected to outer door 36.

is closed against the inner door 34, an air plenum chamber is formed between the two doors.

A fan structure is provided for supplying air for combustion by the burner and is illustrated as mounted directly on the outer door 36 of the boiler structure. The fan structure may be such as described by McClure in US. Patent No. 2,835,320 patented May 20, 1958, entitled Fan Structure and may include, for example, a fan wheel mounted directly on the shaft of driving motor 37. Motor 37 is in turn mounted upon a support A screened air entrance box 39 communicates interiorly for supplying air to the fan structure.

Fire tube 32 is illustrated a being mounted and secured through tube sheet 27 by welding indicated by reference numeral 42, although other means may be used as desired. Fire tube 32 has an open end 43 for receiving combustion gases from burner 33. A closed end 44 of fire tube 32 is suspended within chamber 29 adjacent head or tube sheet 28. End 44 is supported from the bottom of boiler shell 14 by a T bar 48 secured to fire tube 32 and slidable between a pair of upwardly and inwardly turned flanges 41 secured to shell 14. Combustion gas return conduits 45, one to either side of furnace tube 32, are provided for returning combustion gases from adjacent end 44 to a front tube sheet manifold 46. Front tube sheet manifold 46 is defined between inner door 34, head or tube sheet 27 and shell 14 and is isolated generally from the interior of fire tube 32 at end 43. Heat exchange tubes 31 communicate with manifold 46 and a second manifold 47, defined between tube sheet 28 and rear door ascmbly 22 and insulation 26 within shell 14. Exhaust outlet 116 communicates with manifold 47.

When furnace or fire tube 32 is heated or cooled, it is free to expand or contract lengthwise independently of shell 14. The support T bar 48 rides within the partially closed channel formed by flanges 41 with the arms of bar 40 extending laterally between the inward turned portions of flanges 41 and the bottom of shell 14. The T bar 40 supports furnace 32 against bouyancy in an upward direction, flanges 41 acting as stops, and against weight in a downward direction, shell 14 limiting downward movement.

The wet back portion or passage 49 'of the boiler is defined between end 44 of fire tube 32 and tube sheet 28. Because fire tube 32 is not attached by end 44 to tube sheet 28, completely free flow is provided through the wet back passage 49. Thus, as liquid within the boiler chamber 29 is heated by passage of combustion gases from burner 33 through fire tube 32 and against end 44, the water is free to circulate and absorb additional heat in passage 49, the water in passage 49 also acting to insulate sheet 28 and door assembly 22 from the heat of fire tube.

As the combustion gases reach end 44 of tube 32, they have completed their first pass through chamber 29 and are thereupon redirected by return conduits 45 to manifold 46, thence through tubes 31 to manifold 47 for exhausting through outlet 16. Heated water or steam is withdrawn through outlet 15.

The furnace 32 extends within chamber 29 freely in relation to shell 14, shell 14 being secured through tube sheet 17 directly to only one end of furnace 32 and return tubes 45. In operation, as furnace 32 is heated and expands, return conduits 45 should be permitted to expand in like manner. Although, the circumferential corrugations 50 along the length of furnace 32 may permit an accordion eflect in furnace 32 and absorption of some of the lengthwise expansion, in the illustrated form it is preferred that the metal in the first pass tube or furnace 32 and the second pass tubes 45 be at the same temperature so that the expansion lengthwise is identical. T increase the temperature of the second pass tubes to more nearly that of the first pass tubes in the form illustrated in FIGURE 1, tubes 45 include dimples or bosses 51 on the interior surfaces thereof. Such dim les or bosses provide turbulent flow within tubes 45 to increase the heat absorbed by tubes 45 to bring them more nearly to the same temperature as furnace 32. The dimples 51, it will be noted, are provided on the sides of tubes 45 as opposed to the top or bottom thereof for better drainage of condensed liquids from tubes 45. Such bosses or dimples are more fully described in co-pending application Serial No. 291,674 filed July 1, 1963, entitled Heat Exchange Apparatus of Cleaver et al., assigned to the same assignee. It is also advantageous to provide such dimples as illustrated at 52 in the sides of heat exchange tubes 31 for the third pass through chamber 29 for better heat exchange.

In a modified and preferred form of second pass tube, i.e. tube 45, the tubes are provided with longitudinal internal fins 56 as illustrated in FIGURE 3 for increasing the surface area for heat absorption to increase the temperature of the tubes. The fins 56 (or the bosses 51) on the interior surface of tubes 45, increasing the temperature of tubes 45, serve also to increase the heat exchange to the surrounding fluid.

Considering the design of second pass tube shown in FIGURE 3, this design also provides expansion characteristics similar to the first pass or furnace tube. When using a tube 45 with the longitudinal internal fins 56, the furnace wall temperature and return tube temperature are approximately the same with respect to any longitudinal expansion results. For example, in the illustrated structure using the tube of FIGURE 3, where the average temperature of the furnace tube wall is 421 F., the wall temperature of second pass tubes 45 is about 383 F. and the temperature of the fins 56 at their mid-point in height (along a radius of tube 45) is about 458 F. Thus, the average values of furnace wall temperature and overall second pass tube temperature are approximately equal, resulting in equal expansion lengthwise assuming similar materials of construction.

It will be noted with reference to FIGURE 1, that a plurality of small stay members 58 are provided interiorly of chamber 29 and secured to tube sheet 28 and to the interior surface of shell 14. Stays 58 illustrate the positioning of stays for holding the tube sheet 28 against warping under pressure where such additional holding is desired. However, the tubes 31 extend between the tube sheets 28 and 27; tubes 31, which surround furnace tube 32, may be sufficient to retain the tube sheets 27 and 28 against internal expansion without stays or with minimal stays.

In order to increase the ability of tubes 31 to hold tube sheets 27 and 28 against internal pressures within the boiler chamber and to minimize the number of stays 58, in a preferred form of boiler, some of the heat exchange tubes 31 are canted as indicated by reference numeral 31a. If all tubes 31 were parallel, on the sides of furnace 32 where return tubes 45 project it would be impossible to attach tubes to the rear tube sheet 28 in the area aligned with or behind tubes 45, the area 60 laterally of the outline of furnace 32 on tube sheet 28. The number of stays 58 required to hold a tube sheet increases with an increase in number of large gaps between tube ends on the tube sheet. In the preferred arrangement, the number of stays is held to a minimum by placing tube ends within gaps, e.g. area 60, which would be left by securing parallel tubes to the rear tube sheet. Generally straight tubes 31a, attached at one end to tube sheet 27, are canted to by-pass tube 45 and the other ends of tubes 31a are secured to tube sheet 28 in area (it) behind tubes 45. In this manner, the tube ends can be secured on tube sheet 28 so as to closely surround the circular outline of furnace 32 on tube sheet 28 and to hold completely around furnace 32 without obstruction by return tubes 45. Also in the preferred structure, the tubes 31a are canted in at least two directions,

canting in from opposite sides of tubes 45 for attachment to tube sheet 28.

As a more particular example of boiler structure in accordance herewith, in an advantageous form the second pass tubes have a diameter at least one-quarter of the diameter of the first pass single furnace tube and a plurality of such second pass tubes is provided. As a preferred example, tube 32 may be 30 inches in diameter and tubes 45 may be inches in diameter. The fins 56 within tubes 45, Where such fins are employed, may be, for example inch wide and one inch high and may extend longitudinally on the interior surface of, for example, a 10 inch second pass tube. It is an advantageous feature with respect to the second pass tubes that they are of sufficient diameter to facilitate cleaning, i.e. they are of sufiicient size to permit manual cleaning. This is of importance particularly because much of the deposits, e.g. particularly vanadium pentoxide, which is deposited at temperatures of from about 1200 to about 2,000 F., will be deposited within the second pass tube which are of suflicient temperature for such deposition, not in the very small third pass tubes where the temperature has decreased sutficiently to prevent undue forma tion of such deposits.

The present invention has provided a boiler structure of economical construction and maintenance. The third pass heat exchange tubes 31 are readily accessiblefrom the rear tube sheet 28 without the necessity of having to move bulky refractory material on a hinge or otherwise. The structure includes minimal support of the rear tube sheet 28 by extraneous stays. The first and second passes of the flow system are expansible without transmittlng the expansion force to the boiler shell or tube sheet structure. The system permits the use of flat heads or tube sheets in the boiler structure. The flat heads lend themselves to easier and more economical attachment of flues and the like than would be so with a cured or dished head member. The heat exchange tubes 31 may be rolled in place through the flat tube sheet members and need not be welded. The flue holes may more readily be formed by drilling or the like in the fiat tube sheet or head members. The floating heating element or furnace tube, attached at one end, may substantially reduce or eliminate problems from thermal shock caused by pumping ice water into chamber 29, which may often happen where supply lines to the boiler are run over long distances.

I claim:

1. A boiler for heating a liquid medium by heat exchange with combustion gases by means of a plurality of passes of combustion gases comprising a shell, opposing front and rear tube sheets mounted within said shell and defining a chamber therebetween for the liquid to be heated, and manifolds beyond each tube sheet within the shell, furnace tube means containing a burner adjacent one end thereof, said furnace tu'be means being supported by one end and mounted to the front tube sheet and said furnace tube means extending into said chamber to a position spaced from and in proximity with the rear tube sheet to define a free flow wet back passage between the other end of the furnace tube and rear tube sheet, at least two second pass tubes each opening at one end to said furnace tube adjacent said other end thereof and said second pass tubes opening at their other ends to the manifold beyond the front tube sheet, each of said second pass tubes being provided with means to increase the heat transfer between hot gases passing therethrough and the Walls thereof to cause the same to expand longitudinally the same amount as said furnace tube with temperature changes from ambient shutdown temperature up to and including maximum operating temperature of the boiler, and a plurality of relatively small heat exchange tubes extending from the front tube sheet manifold to the rear tube sheet manifold.

2. A boiler for heating a liquid medium by heat eX- change with combustion gases by means of a plurality of passes of combustion gases comprising a shell, opposing front and rear tube sheets mounted within said shell and defining a chamber therebetween for the liquid to be heated, and manifolds beyond each tube sheet within the shell, furnace tube means containing a burner adjacent one end thereof, said furnace tube means being supported by one end and mounted to the front tube sheet and said furnace tube means extending into said chamber to a position spaced from and in proximity with the rear tube sheet to define a free flow wet back passage between the other end of the furnace tube and rear tube sheet, at least two second pass tubes each having a diameter of at least one-quarter the diameter of the furnace tube with the second pass tubes each opening at one end to said furnace tube adjacent said other end thereof and said second pass tubes opening at their other ends to the manifold beyond the front tube sheet, each of said second pass tubes being provided with means to increase the heat transfer between hot gases passing therethrough and the walls thereof to cause the same to expand longitudinally the same amount as said furnace tube with temperature changes from ambient shutdown temperature up to and including maximum operating temperature of the boiler, and a plurality of relatively small heat exchange tubes extending from the front tube sheet manifold to the rear tube sheet manifold.

3. A boiler for heating a liquid medium by heat exchange with combustion gases by means of a plurality of passes of combustion gases comprising a shell, opposing front and rear tube sheets mounted within said shell and defining a chamber therebetween for the liquid to be heated, and manifolds beyond each tube sheet Within the shell, furnace tube means containing a burner adjacent one end thereof, said furnace tube means being supported by one end and mounted to the front tube sheet and said furnace tube means extending into said chamber to a position spaced from and in proximity with the rear tube sheet to define a free flow wet back passage between the other end of the furnace tube and rear tube sheet, at least two second pass tubes each having a diameter of at least one-quarter the diameter of the furnace tube with the second pass tubes each opening at one end to said furnace tube adjacent said other end thereof and said second pass tubes opening at their other ends to the manifold beyond the front tube sheet, each of said second pass tubes being provided with heat convection fins extending longitudinally along the interior surfaces thereof to increase the heat transfer between hot gases passing therethrough and the walls thereof to cause the same to expand longitudinally the same amount as said furnace: tube with temperature changes from ambient shutdown temperature up to and including maximum operating temperature of the boiler, and a plurality of relatively small heat exchange tubes extending from the front tube sheet manifold to the rear tube sheet manifold.

4. A boiler for heating a liquid medium by heat exchange with combustion gases by means of a plurality of passes of combustion gases comprising a shell, opposing front and rear tube sheets mounted within said shell and defining a chamber therebetween for the liquid to be heated, and manifolds beyond each tube sheet within the shell, furnace tube means containing a burner adjacent one end thereof, said furnace tube means being supported by one end and mounted to the front tube sheet and said furnace tube means extending into said chamber to a position spaced from and in proximity with the rear tube sheet to define a free flow wet back passage between the other end of the furnace tube and rear tube sheet, two second pass tubes each having a diameter of at least one-quarter the diameter of the furnace tube with the second pass tubes each opening at one end to said furnace tube adjacent said other end thereof and said second pass tubes opening at their other ends to the manifold beyond the front tube sheet, each of said second pass tubes being provided with heat convection fins extending longitudinally along the interior surfaces thereof to increase the heat transfer between hot gases passing therethrough and the walls thereof to cause the same to expand longitudinally the same amount as said furnace tube with temperature changes from ambient shutdown temperature up to and including maximum operating temperature of the boiler, a plurality of relatively small heat exchange tubes extending from the front tube sheet manifold to the rear tube sheet manifold and comprising the major structural support for the rear tube sheet interiorly of the periphery thereof, and an exhaust outlet for exhausting combustion gases from said rear tube sheet manifold.

References Cited by the Examiner UNITED STATES PATENTS Taylor 122149 Taylor 122149 Nigh 122-149 Pounce 165133 Walker et al. 122-149 Sullivan 122149 De Porag 122149 Brown et al. 122-149 ROBERT A. OLEARY, Primary Examiner.

KENNETH W. SPRAGUE, Examiner. 

1. A BOILER FOR HEATING A LIQUID MEDIUM BY HEAT EXCHANGE WITH COMBUSTION GASES BY MEANS OF A PLURALITY OF PASSES OF COMBUSTION GASES COMPRISING A SHELL, OPPOSING FRONT AND REAR TUBES SHEET MOUNTED WITHIN SAID SHELL AND DEFINING A CHAMBER THEREBETWEEN FOR THE LIQUID TO BE HEATED, AND MANIFOLDS BEYOND EACH TUBE SHEET WITHIN THE SHELL, FURNACE TUBE MEANS CONTAINING A BURNER ADJACENT ONE END THEREOF, SAID FURNACE TUBE MEANS BEING SUPPORTED BY ONE END AND MOUNTED TO THE FRONT TUBE SHEET AND SAID FURNACE TUBE MEANS EXTENDING INTO SAID CHAMBER TO A POSITION SPACED FROM AND IN PROXIMITY WITH THE REAR TUBE SHEET TO DEFINE A FREE FLOW WET BACK PASSAGE BETWEEN THE OTHER END OF THE FURNACE TUBE AND REAR TUBE SHEET, AT LEAST TWO SECOND PASS TUBES EACH OPENING AT ONE END TO SAID FURNACE TUBE ADJACENT SAID OTHER END THEREOF AND SAID SECOND PASS TUBES OPENING AT THEIR OTHER ENDS TO THE MANIFOLD BEYOND THE FRONT TUBE SHEET, EACH OF SAID SECOND 